<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input:first-child {
            border: 1px solid green;
        }
    </style>
</head>

<body>
    <div>
        <input type="text" />
        <input type="text" />
        <input type="text" />
    </div>
</body>

</html>